One general type of liquid paint spray gun uses pressurized air for atomizing the paint and for shaping the pattern of the atomized paint. Generally, a relatively low flow of compressed air at between 25 psig and 90 psig has been used for paint atomization. This relatively high pressure air is quite effective at paint atomization. However, the high air pressure imparts a high velocity to the atomized paint. As a consequence of the high velocity, a significant portion of the atomized paint fails to deposit on the workpiece being coated. Such paint ends up in the surrounding environment. In recent years, there has been an increased emphases on using high volume low pressure (HVLP) air for paint atomization because it can produce a soft spray with a resulting higher transfer of the atomized paint to the workpiece. In the industry, HVLP has generally been understood as a flow of air at no greater than 10 psig. HVLP guns are designed either to operate from a low pressure turbine which supplies the pressurized air at no greater than 10 psig or to operate from a high pressure air line. When operated from a high pressure air line, calibrated orifices internal to the spray gun have been used to drop the air pressure to the desired level.
Generally, a paint cup is attached directly to a hand held spray gun barrel. When the gun is of the type operated from high pressure air, typically the flow of atomization air through a nozzle assembly is used to produce suction which causes the paint to flow from the cup to the fluid tip. When higher viscosity materials are being atomized, it is sometimes necessary to pressurize the paint cup to achieve adequate paint flow.
For HVLP spray guns, pressurized paint feed is generally required because the low air pressure is inadequate to establish the desired paint flow from the cup to the fluid tip. However, there have been problems in achieving the desired paint cup pressure from a low pressure system. The air passages in the spray gun normally cannot be connected directly to pressurize the paint cup because the air pressure in the gun typically is about 10 psig while the ideal paint cup pressure may be about 2 to 3 psig. It will be noted that the volume of air in the paint cup is essentially static, since the air volume changes only as paint is slowly fed to the gun. Because of the low flow, a simple needle valve will not work for adjusting the paint cup pressure.
One prior art HVLP spray gun has placed an air pressure regulator between the spray gun barrel and the paint cup. Although this can produce the desired cup pressure, it also has some inherent problems. If the gun is tipped to paint a horizontal surface, there is a risk that paint will enter the regulator. Further, laws in some governmental jurisdictions such as California require that the spray gun and cup be cleaned in a closed gun cleaner which reduces the discharge of paint solvent vapors into the environment. The regulator cannot be placed in such gun cleaners and it is not easily removed from the gun and cup assembly.
In another prior art HVLP spray gun of the type operated from a high pressure compressed air source, a regulator is attached to the spray gun handle in series with the compressed air supply hose. The regulator is connected for supplying the desired low air pressure to the paint cup. The controlled low pressure outlet from the regulator is connected through a flexible hose and a barbed fitting on a check valve to the paint cup lid for pressurizing the cup. The check valve reduces the risk of paint entering the regulator when the spray gun and cup assembly is tipped during use. This arrangement functions well for controlling the paint feed pressure. However, this arrangement also has disadvantages. The regulator body must have an unregulated passage to handle the full air flow to the gun plus the regulated port to the cup. Further, the pressure regulator adds about 7 to 8 cm (about 3 inches) to the length of the handle. Again, the regulator is not quickly removable for placing the spray gun in a closed gun cleaner. If quick connect fittings are used to permit quick removal of the regulator for cleaning, the end of the paint hose is further removed from the spray gun handle. Spacing the point of attachment of the flexible air hose 7 to 8 cm or more below the gun handle has a significant adverse affect on the balance of the spray gun during use. For good maneuverability, the flexible air hose should be attached directly to the gun handle. Further, spacing the point of attachment 7 to 8 cm below the handle can prevent setting the gun upright on a bench and also can cause an imbalance due to the distance from the center of gravity of the gun and cup. Consequently, the gun and cup can tip easily.
There is a need for an HVLP spray gun having a regulated pressure feed paint cup in which the pressure regulator is easily disconnected from the gun to permit cleaning the gun in a closed gun cleaner and in which the regulator does not adversely affect the maneuverability and balance of the gun during use.